In an internal combustion engine having a carburetor mounted in an automotive vehicle, the amount of air and fuel supplied to the engine must be accurately controlled depending on operating conditions of the engine in order to maintain a proper matching relationship therebetween, so as to effectively purify exhaust gases from the engine and to operate the engine at the highest possible efficiency at all times.
A three-way catalyst is commonly used for the desired purification of exhaust gases from an engine of this type. In such an engine, the output signal from a gas sensor, for example, an O.sub.2 sensor disposed in the exhaust system, is fed back to control the air-fuel ratio of the air-fuel mixture supplied to the engine so as to maintain it at the value of 14.7 which is the stoichiometric air-fuel ratio. However, this air-fuel ratio of 14.7 is not always optimum from the viewpoint of efficiency of engine operation and also from the viewpoint of fuel consumption. Therefore, in order to ensure the highest possible efficiency of engine operation and the lowest possible fuel consumption in the engine, it is necessary to control the air-fuel ratio rather than only the feedback control based on the output signal from the O.sub.2 sensor, while, at the same time, carrying out the desired purification of engine exhaust gases.
An electronically controlled carburetor for an internal combustion engine having a carburetor is proposed in, for example, Japanese Patent Applicaion Laid-open No. 50126/73, in which a computer is employed to arithmetrically calculate the air-fuel ratio on the basis of the variables of operating conditions of the engine so as to control the air-fuel ratio. In the proposed electronically controlled carburetor, a solenoid-operated valve is disposed in the fuel system leading to the carburetor, and the computer makes necessary calculations on the basis of the sensed variables of the operation conditions of the engine to control the valve, thereby suitably modifying the air-fuel ratio as determined by the setting of the carburetor. While this electronic control of the air-fuel ratio is effective, it is now necessary to attain the desired air-fuel ratio control in a simpler and more accurate manner than hitherto, as the capacity of modern memory elements becomes larger and larger.
On the other hand, it is necessary to appropriately control the rate of exhaust gas recirculation depending on the operating conditions of the engine, so that exhaust gases from the engine can be efficiently purified, by way of exhaust gas recirculation, which is one of the measures employed for the purification of engine exhaust gases.